High quality image acquisition in electron microscopes requires careful alignment of the electron beam and precise focusing for optimized image contrast and fine detail. In the past, physical features and characteristics of the electron-microscope have been important and used to perform the alignment. The electron beam is aligned by using electromagnetic devices. A misaligned electron beam results in artifacts (ripples), blurriness in the image, and loss of information on fine details.
An important feature of the present invention is that the method automatically corrects for lens astigmatism during the alignment process by using only image data and without relying on complicated and cumbersome features of the microscope itself. The method of the present invention provides a solution to the above-outlined problems. More particularly, the method is for automatic astigmatism correction in one direction through a set of lenses. Of course, the present invention is not limited to correcting in only one direction because the correction can also be done in many directions simultaneously such as both the x- and y-directions. A first image is provided at a first stigmator setting of a lens. Preferably, the image is under-focused. Based on the first image, a calculating device calculates a first Fourier spectrum image. The distribution and direction of pixels of the Fourier spectrum image are determined by calculating a first vector corresponding to the main direction and extent of the bright pixels, and a second vector being perpendicular to the first vector and corresponding to the extent in that direction. The first vector is compared with the second vector. The set of lenses is changed from a first stigmator setting to a second stigmator setting to provide a second under-focused image. Based on the second image, the second corresponding Fourier spectrum image is calculated. The distribution and direction of pixels of the second Fourier spectrum image is determined by calculating a third vector and a fourth vector. The third vector is compared with the fourth vector. When the first vector is more similar to the second vector than the third vector is to the fourth vector the first image is selected as being more round than the second image. When the third vector is more similar to the fourth vector than the first vector is to the second vector then the second image is selected as being more round than the first image. The stigmator settings providing the Fourier spectrum with the most round Fourier spectrum is what is strived and searched for.
The method further includes the step of calculating grey-weighted moments of a circular Fourier spectrum image as a means of measuring the direction and extent of the intensity distribution.
In another embodiment, a first ratio of eigen-vectors of the first Fourier spectrum image is compared with a second ratio of eigen-vectors of the second Fourier spectrum image.
The image with the lowest ratio is selected.
The x-stigmator and y-stigmator settings are changed to the stigmator settings that correspond to the image with the lowest ratio.
The x-stigmator and the y-stigmator settings can also be simultaneously changed.
In yet another embodiment, the stigmator setting that minimizes the elongation value of the Fourier spectrum image is searched for.
The first and the second images are set to an under-focus or an over-focus.